Caught in Swallowtails: Discovery of Two Swallowtail Image Formations in MS 0451.6-0305

TL;DR

This paper reports the discovery of two swallowtail image formations behind the MS0451.6-0305 galaxy cluster using JWST-NIRCam imaging, at redshifts $z=2.91$ and $z=6.70$. Using the Zitrin-Analytic strong-lensing framework, the authors reveal higher-order swallowtail caustics that magnify compact substructures in background galaxies to sub-parsec scales, with the $z=2.91$ knots reaching point-source magnifications of $\\gtrsim 300$ and inferred source-plane radii $\\lesssim 1$ pc, while the $z=6.70$ arc shows magnifications in the $\\sim 20-200$ range and radii $\\sim 0.8-18.5$ pc. The work emphasizes the role of swallowtail caustics in probing fine-grained structure and the cluster mass distribution, and it discusses the limitations and uncertainties in magnification by exploring alternative knot associations and lens models. A multi-epoch transient search finds no significant lensed transients in these arcs, but the demonstrated magnification and resolution open avenues for future studies of sub-parsec scale features and potential lensed stellar events in similarly structured clusters.

Abstract

We report the discovery of two swallowtail image formations at $z=2.91$ and $z=6.70$ behind the galaxy cluster MS 0451.6-0305 in JWST-NIRCam imaging. We find that in both of the above lensed systems, the complex image morphology cannot be reproduced by simple fold/cusp caustics, and detailed lens modeling reveals higher-order swallowtail caustic configurations. In the $z=2.91$ lens system, a small part of the source galaxy (which itself is part of a galaxy group) containing atleast two compact knots sits inside the swallowtail caustic, producing a quadruply imaged arc. At two of the image positions of these knots, we infer point source magnifications of $\gtrsim 300$, implying lensing-corrected effective radii of $\lesssim 0.8-1.5$ pc. The $z=6.70$ system exhibits even more complex image formation. We therefore only use the most confidently identified counter-images of knots in this system as constraints in our lens modeling. The resulting model predicts magnifications $\sim20-200$ and lensing-corrected effective radii of $\lesssim 0.8-18.5$ pc for various knots. Together, these two systems represent the first example of observations of multiple swallowtail image formations in a single galaxy cluster and demonstrate the ability of swallowtail caustics to magnify individual substructures at sub-parsec scales, from intermediate redshifts to the first billion years of the Universe.

Figures (4)

• Figure 1: JWST false color image ($R={\rm F356W+F444W}$, $G={\rm F200W+F277W}$, $B={\rm F115W+F150W}$) of MS0451 ($z=0.55$) galaxy cluster. The yellow curves show the critical curves for a source at redshift $z=6.70$, corresponding to Zitrin-Analytic lens model. The lensed images are encircled with thin, dashed green circles, and their positions are further highlighted by green arrows. We did not mark systems 10.1 to 10.5, as they lie very close to each other. The knots in system 19 are also not marked for the same reason. Both systems 10 and 19 correspond to the swallowtail image formations. For a zoomed-in view of these arcs, we refer readers to Fig. \ref{['fig:swallow_291']} and Fig. \ref{['fig:swallow_6p7']} below.
• Figure 2: Color-magnitude diagram for objects in MS0451. The black points represent all objects detected in the HST-ACS-F814W filter. The green and red points show the spectroscopic and photometric cluster members, respectively. The cluster red sequence is shown in the gray shaded region. The vertical dashed curve highlights the limiting magnitude for cluster members selected using the red sequence analysis.
• Figure 3: Swallowtail image formation at $z=2.91$. Left panel shows the false color image of the cluster with blue and yellow dashed curves representing the critical curve for $z=2.85$ and $z_s=2.91$, respectively. The quadruply imaged knots are marked by colored arrows. Middle panel represents the corresponding magnification map with different colored (same as the arrow colors in the left panel) stars marking the observed positions of the quadruply imaged knots. Right panel shows the corresponding caustic structure in the source plane. The solid and dashed curves in blue and black show tangential and radial critical caustics at $z=2.85$ and $z=2.91$, respectively, showing the evolution of caustics close to the source position(s). The three stars show the barycenter source position for the multiple image systems shown in the left and middle panels. The three inset panels display a zoomed-in view of the regions near each source position. Note that the scale in the right panel shows the proper distance in the source plane.
• Figure 4: Swallowtail image formation at $z=6.70$. Left panel shows the false color image of the cluster with yellow dashed curves representing the critical curve for $z_s=6.70$. Different systems and knots are shown in different colors. We note that 19.K1/K2/K3/K4/K5 are not counter-images of each other; instead, they are individual knots. See Sec. \ref{['sec:swallow_6p7']} for more details. Middle panel shows the corresponding magnification map with different colored (same color scheme as the left panel) stars representing the observed positions of the different knots. Right panel shows the caustic structure in the source plane with solid and dashed curves representing the tangential and radial caustics, respectively. The stars indicate the barycenter position of different multiple-image systems shown in the left panel. Note that the scale in the right panel shows the proper distance in the source plane.